Liners of the type in question here have a certain wall thickness and function as a cushioning intermediate layer that is formed between the amputation stump and the inner wall of a prosthesis socket and is adapted or adapts to the amputation stump. The prosthesis socket is part of a prosthesis that replaces the amputated part of an extremity of the patient.
Transmission of electrical signals between the amputation stump and the outside of the liner may be considered for a number of reasons. Thus, it may be useful to transmit electrical signals from the skin of the amputation stump to the outside, in order to control the function of the prosthesis. In this case, the electrodes can be myoelectric electrodes which pick up muscle contraction signals at suitable points of the amputation stump, as a result of which corresponding prosthetic limbs can be controlled. Myoelectric control of prostheses is known particularly for prosthetic arms and hands but can also be used for prosthetic legs and feet.
It may also be useful to electrically determine the surface resistance of the skin of the amputation stump by measuring a current flow between two or more electrodes or electrode sections. It is in this way possible, for example, to determine whether the skin of the amputation stump within the liner transpires, which can adversely affect the fit of the liner on the amputation stump, hence the fit of the prosthesis. It is also possible to use electrodes to determine the contact pressure of the amputation stump on the inner wall of the liner, so as to be able to react, for example, to mass shrinkage of the amputation stump during the period the prosthesis is worn.
Conversely, it may be useful to transmit electrical signals from the outside of the liner to the skin of the amputation stump, for example in order to excite a muscle contraction of the amputation stump when the prosthesis wearer is located for some time in a passive position, for example a seated position.
U.S. Pat. No. 5,443,525 discloses a liner that is intended to receive myoelectric electrodes. For this purpose, a non-metallic, flexible and soft pad, in which a large number of discrete conducting electrodes are located, is bonded into a window of the prosthesis socket. The liner is preferably made of silicone, a non-conducting flexible plastic. The electrodes can be formed from a mixture of silicone and carbon or of silicone and silver, the electrodes being surrounded in each case by non-conducting silicone. The electrode arrangement is thus bonded via the pad onto the inner face of the liner and is accessible through the window of the liner, such that the myoelectric signals picked up by the electrodes can be conveyed through the window to the outside for evaluation or control. This arrangement is expensive to produce and provides limited wearing comfort. In addition, the window of the liner requires special sealing if the liner, as is often the case, has to be made airtight, in order to hold the liner on the amputation stump with the aid of an underpressure created in the interior of the liner. The underpressure has to be maintained by the liner counter to the weight of the moved prosthesis.